1. Field of the Invention
The present invention relates to a fully automatic driverless straddle carrier for transporting and stacking freight containers. More particularly, the invention relates to a straddle carrier which is equipped which several different sensor systems for vehicle detection and navigation, the signals of which being evaluated and monitored in an electronic sensor fusion system and the current coordinates of the location determined thereby then being transmitted to a system of electronic controls for automatically steering, driving and positioning the vehicle on present paths stored in an electronic path control, laser scanners being arranged at the vehicle for automatically guiding the vehicle over a container stack.
2. The Prior Art
Straddle carriers are used all over the world in seaports and container terminals for transporting and stacking containers. As shown in U.S. Design Pat. No. D 526,932 S, they consist of two traveling gears with wheels running to the left and to right of the container stack, and of vertical supports placed thereon forming together with the machine scaffold positioned thereon a portal. This portal travels over the containers and lifts them via its lifting gear or puts them down. The straddle carriers have a diesel-electric or a diesel-hydraulic drive and in most cases an electrohydraulic steering mechanism. After presetting the deflection of the steering wheel by the driver, a steering computer controls the hydraulic proportional valves which in turn cause the deflection of the wheels via hydraulic cylinders. Also driverless fully automatic straddle carriers are known. They use in most cases the commonly known processes used for the navigation of all autonomous driverless industrial trucks, in particular individual systems or combinations of absolute and relative navigation sensors.
The following absolute navigation systems are known:                satellite navigation (differential global positioning system (GPS, DGPS))        radar navigation (with a radar device having a rotating antenna on the roof of the vehicle and stationary passive reflectors in the surroundings)        transponder navigation (with transponders or magnets buried in the ground).        
The following relative navigation systems are known:                inertial navigation (gyroscopes, inertial measuring units (IMU))        dead reckoning navigation with odometry (distance measuring by rotary encoders at the wheels) and probably additional measuring of the steering angle via a further sensor at the steering mechanism.        
For a fully automatic steering, driving and positioning, these known navigation systems must allow the detection of the vehicle position very reliably and with an accuracy of one centimeter. The navigation is therefore very complex and the requirements are fulfilled only by a combination of different redundant diversity, absolute and relative navigation processes.
In “Kalmar launches Autoshuttle” WorldCargo News (Jan. 31, 2008) a solution for fully automatic straddle carriers has been described. The system works with transponder navigation according to the FROG technology, Free Ranging On Grid, in which antennas or magnetic-field sensors are placed at the bottom of the vehicle, which detect transponders or magnets buried in the roadway and recalibrate the odometry in this manner.
“An Autonomous Straddle Carrier for Movement of Shipping Containers”, IEEE Robotics & Automation Magazine of September 2007, uses a combination of DGPS (satellite based global positioning system), rotating dwarf waves radar, inertial navigation and odometry. The detection signals of these individual navigation systems are combined in a sensor fusion system consisting of Kalman filters and evaluated in order to obtain a reliable and precise detection signal, because each individual sensor alone would be too unreliable or too inaccurate for a fully automatic driving mode.
In DE 103 36 084 A1 a local position measuring system is described which has a base station located on a mobile object, the position of which is to be determined, a plurality of transponders being distributed around the locality.
This system is also known as “Local Positioning Radar,” but must not be confused with a navigation radar having a rotating antenna on the vehicle.
These known processes for a fully automatic vehicle steering for straddle carriers have some disadvantages.
Solutions as that according to the FROG technology have the disadvantage that considerable time and efforts are required for the installation of the magnets in the ground. Because the FROG antennas at the bottom of the vehicle cannot be wider than the two traveling gears on each vehicle side, i.e. about 70 cm, a very high density of the transponders is required for the case where the odometry drifts off so that at least one transponder or magnet is detected on these 2×70 cm in order to allow a recalibration. Thus, thousands of holes must be drilled into all roadways of the terminal in order to install the magnets in the ground. Besides these substantial efforts the handling of goods is obstructed during the installation.
Solutions with a combination of DGPS with radar have the disadvantage that the mechanics of the dwarf waves navigation radar mounted on the vehicle are expensive and susceptible to faults and the required expenses in electronics in such a radar device are in economic aspects hardly justifiable.
Both solutions have the further disadvantage that the vehicles do not orient themselves at the container stack itself when traveling over a container stack but at external marks, like satellites in the case of DGPS, reflectors in the case of rotating radar, magnets in the case of FROG. By these mechanisms, the vehicle is guided along a merely theoretical path, stored in the electronic control, on which the container stack should be positioned theoretically. When the containers are in reality not exactly placed on this theoretic path, grazing, collisions, damages and thus working interruptions may occur.